Digital interface unit

ABSTRACT

An interface apparatus and method is disclosed which allows communication of digital data messages between a first type of data terminals (10 and 65) employing a message protocol (FIG. 1a) with a &#34;free text-type&#34; message, and other user devices employing other digital message protocols. The invention is adapted to exploit the free text message capability of the first protocol. In accordance with the invention, the incoming user data messages are packed into one or more free text messages of the first message protocol, and conversely &#34;unpack&#34; free text messages from the data terminal employing the first protocol are unpacked and formatted into the message protocol format employed by the user device 65. The invention eliminates the necessity for expensive translation devices which translate the data, and allows a plurality of user data terminals to communicate via a link comprising the data terminals employing the first message protocol.

This application is a continuation of application Ser. No. 07/081,561filed July 31, 1987, now abandoned, which is continuation of Ser. No.06,802,786, filed Nov. 27, 1985, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to interface units for allowing digitalcommunication between digital data devices employing different messageprotocols.

There are presently data communications devices in use today whichrequire the data to be packed into specific message formats for reliabledata transmission and reception. Examples of such devices are terminalswhich are employed for anti-jam secure radio transmission. There aredifferent types of such terminals in use today, each adapted forcommunication of digital data in a specific message format. By way ofexample, the U.S. military employs a tactical radio system known as the"Joint Tactical Information Distribution System" (JTIDS) for tacticalinformation exchange in an anti-jam secure link.

JTIDS employs a communication technique known as Time Division MultipleAccess (TDMA) which permits messages to be sent from numerous terminalson a specified network in a time-sequenced basis. JTIDS is a jamresistant system using spread spectrum techniques and fast frequencyhopping to distribute the transmitted data over a wide frequencybandwidth. Thus, JTIDS information is broadcast omnidirectionally atmany thousands of bits per second and can be received by any JTIDSterminal within range. Information flows directly from many transmittersto many receivers; each JTIDS terminal can select or reject each messageaccording to its need for the information.

The JTIDS may employ one particular data protocol known as Interim JTIDSMessage Set "IJMS", or another protocol known as Tactical Digital Link"TADIL J", comprising an alphabet of types of data messages. Forexample, each IJMS message comprises a header word which defines thetype of message, and eight data words containing the information. Asingle message might comprise information such as track report, fuel andordinance reserves, position, and so on.

Other types of terminals also exist, with data protocols which do notcoincide with those employed by JTIDS. Military examples include ArmyTactical Datalink ATDL-1, TADIL A, and TADIL B. These terminals providesecure transmission capabilities, but are not adapted to providesignificant jamming resistance. In many instances, it would beadvantageous to allow communication between a JTIDS network and anothertactical network such as TADIL B, so as to take advantage of an existingJTIDS network to provide a jamming resistance capability.

Thus, there exists a need to provide a means to allow communicationbetween two or more types of communication terminals employing differentmessage formats. Attempts have been made to meet this need by theprovision of a translator terminal, which receives the informationsignal from one type of communication terminal employing one specificmessage format, and "translates" the intelligence from the terminal intoa second type of message format compatible for utilization by the seconduser. The translation process is understood to involve the decoding ofthe information in one format, and subsequent encoding of the data intothe second type of message format. Such translator devices are subjectto translation losses or errors and are very expensive.

It would therefore be an advance in the art to provide an improved meansfor interfacing between data communications terminals employingdifferent message protocols so as to allow communication between suchterminals, without employing a data translator.

SUMMARY OF THE INVENTION

In accordance with the invention, an interface unit is provided tofacilitate communication between a first digital data terminal employinga link message protocol comprising a free text message type format and aplurality of other digital data terminals (the "users") employing othermessage protocols. The invention exploits the free text capability ofthe link message protocol, wherein the data bits of that message have nopredetermined significance, as is the case with fixed format messages.The interface unit is adapted to pack the incoming digital messages fromthe user devices into "free text" messages in the link message protocol.Thus, such digital messages from the user device may be communicated viaa first interface unit to a first data terminal, and relayed over acommunication link to a second data terminal employing the link messageprotocol for the respective user device. The second data terminal iscoupled to a second interface unit which is adapted to recognize andunpack free text-type messages into the appropriate user messageprotocol. The second interface unit may be coupled to a plurality ofuser devices.

The invention further comprises the method for communicating digitaldata from one or more user device over a data link comprising first andsecond link terminals employing a link message protocol. The linkprotocol includes a first message type wherein the data bits in selectedwords do not have a predetermined significance. In accordance with themethod, user digital data is packed into one or more messages of thefirst message type and and are provided to the first link terminal fortransmission to the second link terminal. The messages received at thesecond link terminal are unpacked from the link protocol and formattedinto the user protocol. This user protocol data is then provided to oneor more user devices which employ the user protocol. Hence, theinvention allows user digital data terminals to communicate via a linkcomprising the link data terminals employing the first message protocol.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent from the following detailed description of anexemplary embodiment thereof, as illustrated in the accompanyingdrawings, in which:

FIG. 1(a) illustrates a first message protocol comprising a free textmessage, and FIG. 1(b) illustrates a second message protocol.

FIG. 2 is a block diagram of a digital data communication link employingthe invention.

FIG. 3 is a block diagram of the presently preferred embodiment of theinterface unit.

FIGS. 4(a) and 4(b) are firmware flow charts of the operation of aninterface unit employing the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention comprises a novel digital data interface apparatusand method. The following description is presented to enable any personskilled in the art to make and use the invention, and is provided in thecontext of a particular application and its requirements. Variousmodifications to the preferred embodiment will be readily apparent tothose skilled in the art, and the generic principles defined herein maybe applied to other embodiments and applications. Thus, the presentinvention is not intended to be limited to the embodiment shown, but isto be accorded the widest scope consistent with the principles and novelfeatures disclosed herein.

The preferred embodiment of the invention is described in the context ofinterfacing with JTIDS type radio terminals. These terminals operatewith a specific digital message protocol, typically either IJMS or TADILJ. Communications between JTIDS terminals are secure and highlyresistant to jamming.

Other types of digital radio terminals are in use, such as the TADIL Aand TADIL C terminals; these terminals are secure but not resistant tojamming. It is often desirable to have the ability to interfaceterminals such as the TADIL A or TADIL C terminals to a JTIDS terminalso as to take advantage of an existing jam resistant communicationsnetwork when the message is being relayed. However, the differencebetween the message protocol employed by the respective terminalsprohibits direct communications between them.

A characteristic of the IJMS and TADIL J message protocols, in additionto fixed format messages, is the provision of a "free text" message.That is, the terminal is adapted to recognize an incoming user messagewhich is "tagged" as a free text message, and to format that message sothat it is transmitted verbatim to the receiving JTIDS terminal fordecoding of the free text message. In effect, the data bits of a freetext message are considered as blanks, with no predeterminedsignificance as is the case for fixed format messages. The provision ofa free text message in a message protocol is very common, and is notlimited to the IJMS and TADIL J protocols. Moreover, the use of the term"message protocol" as used herein means message protocols such as, forexample, IJMS, TADIL etc., as distinguished from signal protocols suchas, serial parallel, etc.

The universal IJMS message protocol is illustrated by way of example inFIG. 1(a), and comprises an alphabet of messages consisting of 9 digitalwords each 32 bits in length. The IJMS language comprises a number ofdifferent message types, each fulfilling a particular tacticalrequirement. Specifically the first word of each message block is the"header" word, which includes bit locations whose values define theparticular message type. For example, bit positions 1 through 3 in theheader word can define the type of message to follow such as, forexample, track report, position, mission status, free text, etc. Thestandard face text message can be further defined as non-error-codedinformation or error coded information, bit position 4 can indicate ifthe message has been relayed or received directly from the sender. Thenext bit positions 5 through 19 can be used to identify the messagesource by its individual code. Bit position 20-22 specify the categoryof message such as a track report message, free text, etc. Bit positionsposition 24 through 26 specify the subcategory such as air track or seatrack, etc. Bit positions 27 through 30 label the message such as byspecifying which one of up to sixteen air tracks are being sent, or inthe case of a free text message, that it is in TADIL-A, TADIL-B, etc.Bit position 31 can identify a special processor, and bit position 32can identify whether the message is a training purpose message. Ofcourse, the above is merely an exemplary format for the header word andother formats could be used.

As indicated above, one of the message types is the "free text" message.In the free text message, the last eight 32-bit-long words are datawords which comprise "blank bits," i.e., the bit values are not assignedany particular intelligence function. These blank bits are available tohave bits of information comprising the message to-be-transmitted packedinto them.

The preferred embodiment of the invention exploits the "free text"message capability of the IJMS or TADIL J protocol in the communicationof digital data, and will first be generally described in connectionwith the communication link block diagram of FIG. 2. The inventioncomprises user devices such as digital communication devices 10 and 65each of which comprises a digital radio terminal which employs anon-IJMS message protocol, By way of example, the first such device 10may utilize the TADIL-A protocol illustrated in FIG. 1(b), wherein eachmessage comprises two digital words each 26 bits in length. For thisexample, it may be assumed that users communicate between the twodevices 10 and 65 over communications links (not shown) which are secure(i.e., the data is encrypted) but not resistant to jamming. However, itis desirable to be able to employ the JTIDS terminal link to allowsecure, jam-resistant communication between the two devices 10 and 65.Thus, two terminals 30 and 45 each comprise JTIDS type radio terminalswhich communicate via an rf link comprising two antennas 35 and 40.

As a result of the different message protocols between the user devices10 and 65 and the JTIDS radio terminals 30 and 45, two interface units20 and 50 are provided to respectively interface the devices 10 and 65to the JTIDS terminals 30 and 45. Hard-wire connections 15 and 25respectively couple the first user device 10 to the interface unit 20and the interface unit 25 to JTIDS terminal 30. Similarly, hard-wireconnections 60 and 50 respectively couple the user device 65 to theinterface unit 55, and the interface unit 55 to JTIDS terminal 45.

The interface unit 20 is adapted, in accordance with the invention, to"pack" the user's message from the first user device 10 in a TADILprotocol, for example, into a "free text" message provided in the IJMSprotocol. The interface unit 55 is adapted to "unpack" the IJMS freetext message into the user's TADIL message format for use by the seconddevice 65, i.e., two words, each 26 bits in length. No translation ofthe data, with the concomittant translation losses, is required. Thus,communication between the user devices 10 and 65 via the JTIDS link ismade possible by the interface units 20 and 55.

A general block diagram of a typical interface unit 20 is shown in FIG.3. Respective user devices 90-93 each employing predetermined messageprotocols A, B, C, D are coupled to interface unit 20 by data lines101-104. The user devices may comprise a wide variety of communicationsdevices including, for example, TADIL terminals.

The interface unit 20 comprises a plurality of input/output (I/O)devices 112, 114, and 116, which selectively couple the respective userdevices 91-93 to a microprocessor 120 or to respective communicationlinks 134, 136, 138. Thus, for example, communication link 138 maycomprise a long range over-the-horizon communication link employing HF(high frequency) or VHF (very high frequency) signals. Link 136 maycomprise a short range, UHF link. Link 134 may comprise a modem forconnection to a land line link, such as a telephone network. An I/Odevice 110 couples line 101 to the microprocessor 120 via line 101b. TheI/O device 122 couples the microprocessor 120 to a JTIDS link device 132via lines 124b and 124a.

The actual configuration of the I/O devices illustrated in block form inFIG. 3 are conventional types and will depend on the particular userdevice. For example, a user device may be configured to comply withMIL-STD 1553B, NTDS or the RS232 specifications, which define the userdevice interface, such as the number of wires and the electricalcharacteristics of the signal (polarity, voltage level and the like).

The I/O devices are controlled via a control bus 118 to determine thestatus of the data paths through the I/O devices. Thus, the respectiveI/O devices 112, 114 and 116 may be controlled so as to couple user line104 to either line 130 for coupling to the VHF link 138 or to line 117for coupling to the microprocessor 120 for eventual coupling to theJTIDS communication device 132 to provide the capability of allowingeach use device to communicate with the JTIDS communications link 132through the microprocessor 120, or through other data links notinvolving the JTIDS terminal. Control buses are well known in the artthus, specific details are not described here.

It is to be understood that the structure of the interface unit 20disclosed in FIG. 3 is merely exemplary, and is intended to illustratethe versatility of interface units which may employ the invention.

The microprocessor 120 may comprise any of a number of commerciallyavailable circuits, such as the Motorola 6800 microprocessor. Computerprograms and data adapted to interface each of the respective userdevices to the JTIDS terminal is stored in memory devices such as PROMs(programmable read-only-memories) for access by the microprocessor 120.The microprocessor 120 is programmed to carry out the functionsillustrated in the firmware flow chart of FIGS. 4a and 4b. Typicalcompilers that can be used to generate the computer programs includePascal, Basic, C-Language and Fortran.

The flow diagram of FIG. 4a illustrates the sequence of steps forformatting the user data into IJMS or TADIL J free text messages. Theincoming data from the user devices 90-93 is stored at step 200 in abuffer memory within the microprocessor 120. Whether data from one ormore of user devices 90-93 is coupled to the microprocessor 120 willdepend on the status of the signal on control bus 118, as describedabove with reference to FIG. 3.

At step 205 each respective data protocol type is identified. The datamay comprise several possible protocols A, B, C, or D depending upon thestatus of the signal on control line 118.

The data type may be identified by correlation of the data with thestatus of the control line 118. Alternatively, the data protocol of theuser device(s) may be predetermined, e.g., where all user devices employthe same message protocol. The data type is "tagged" at step 210, 212,215 and 220. As was explained in more detail with reference to FIG. 1a,in the preferred embodiment, a unique digital code is employed toidentify each of the respective user message protocols. Each incominguser message is therefore associated or tagged with a predetermineddigital identifier.

The identifier may be employed to access the appropriate PROM locationsnecessary to carry out the next step 225, wherein the "tagged" data isformatted into IJMS free text messages. During this step, IJMS or TADILJ format messages are generated, comprising nine 32-bit words asdescribed with reference to FIG. 1a. The IJMS message identifier isspecified in the heater word as a free text message at bit positions 27through 30. Additional bits in the header word are reserved for thedigital identifier of the user message protocol type. Other bitlocations in the header word are employed to identify the specific userterminal to which the message is directed and/or from which the messageoriginated. Thus, the microprocessor is programmed to generate an IJMSfree text message, with the header word comprising the free text messageidentifier and the additional information defining the user messageprotocol and specific user device. The user message is then copiedverbatim into the blank 32 bit data words comprising each IJMS message.Thus, for the user message protocol illustrated in FIG. 1(b), two 26-bitwords of each user message are copied verbatim into two of the eight32-bit data words comprising the IJMS or TADIL J message. Of course,whether the user message is densely packed into the available data bitlocations of the IJMS message or spread out over all of the availablewords is a matter of choice.

For example, if a plurality of user messages are input to the interfaceunit 20 from the same user device, the microprocessor 120 may beprogrammed to fill each of the available data bit locations of the freetext message words with the user message data. Thus, the 52 bits of theuser message may be formatted into the 32 bits of the first data word,and the remaining 20 bits formatted into the first 20 bit locations ofthe second data word. An end-of-message identifier may be placed at theend of the message. Alternatively, the microprocessor 120 may beconfigured to pack one user message per IJMS or TADIL J message, and theend of each message denoted by an "end-of-message" identifier. Suchalternatives may be readily implemented by programming and need not bedescribed in further detail.

The formatted IJMS or TADIL J data is then stored at step 230 and thenoutputted through an output buffer at step 235 to the JTIDS terminal asone or more IJMS messages.

The microprocessor 120 when used in the receiver portion of the systemis also adapted to "unpack" data moving in the opposite direction, i.e.,from the JTIDS terminal to the user terminal 65 (FIG. 1). The processfor unpacking the data is illustrated by the flow chart of FIG. 4b. Theincoming data from the JTIDS terminal is coupled into an input storagebuffer of the interface unit 55 (FIG. 2) at step 250. At step 255 adecision is made to determine whether the message is a "free text"message; this is accomplished by comparing the message identifier bitsin the header word with the predetermined free text code stored in themicroprocessor memory. If the message is not a free text message, thenat step 260 the processor determines whether an IJMS or TADIL J user iscoupled to the interface unit and, if so, passes the message to the IJMSor TADIL J user. If there is no IJMS or TADIL J user, then at step 265the message can be discarded.

In the event that the decision at step 255 is positive, i.e., that themessage is a "free text" message, then at step 270 the free text messageis "unpacked." This is achieved by detecting the type of message fromthe message protocol type information set forth in the message headerand subsequently processing the message to convert the message from theIJMS or TADIL J protocol to the appropriate user protocol such asTADIL-A, for example. Thus, for the example discussed above, the first26 bits in the first data word of the IJMS or TADIL J message areemployed for the first word of the user message and the last 6 bits ofthe first data word and first 20 bits of the second data word used forthe second word of the user message.

At step 275 the particular user 65 (FIG. 2) to whom the message isdirected is detected, again from the user identification bits in themessage header word, and directed to that user.

There has been described an apparatus and method for interfacing onedigital data terminal employing a message protocol comprising a freetext message to another digital terminal employing a different messageprotocol. The invention allows digital data communication between suchterminals without the need for data translation.

It is understood that the above-described embodiment is merelyillustrative of the many possible specific embodiments which canrepresent principles of the present invention. Numerous and varied otherarrangements can readily be devised in accordance with these principlesby those skilled in the art without departing from the spirit and scopeof the invention.

What is claimed is:
 1. Digital communication apparatus for use with acommunications link, said communications link having a message protocolwhich includes a free text message protocol, said digital communicationsapparatus allowing transfer of data, over said communications link,between digital communications terminals of a predetermined type havinga fixed format message protocol which is incompatible with the messageprotocol of said communications link, which comprises:first means,adapted to receive a fixed format digital message having a fixed formatmessage protocol from one of said digital communications terminals ofsaid predetermined type, for packing said fixed format digital messageinto a free text digital message having a free text message protocolcompatible with the free text message protocol of said communicationslink; and second means, adapted to receive a free text digital messagehaving a fixed format digital message contained therein from saidcommunications link, for unpacking said fixed format digital messagefrom said free text digital message, said second means being adapted totransmit said fixed format message to one of said digital communicationsterminals of said predetermined type.
 2. The digital communicationapparatus of claim 1 wherein said first means comprises:means forstoring said fixed format digital message received from one of saiddigital communications terminals; means for identifying the messageprotocol type of said fixed format digital message; means for taggingsaid fixed format digital message with digital identifiers indicatingthe fixed format message protocol type, that this message is to betransmitted as a free text message, and the terminal type to which saidfixed format message is to be transmitted; means for formatting saidtagged fixed format digital message into said predetermined free textmessage format compatible with said communications link; and means forstoring said formatted free text message.
 3. The digital communicationapparatus of claim 2 wherein said means for formatting comprises:meansfor creating a free text header word which includes said digitalidentifiers; and means for copying said fixed format message intounoccupied digital words comprising said free text message.
 4. Thedigital communication apparatus of claim 2 wherein said second meanscomprises:means for storing a free text message received from saidcommunications link; means for detecting the receipt of said free textmessage from said communications link; means for uniformatting said freetext message to extract said fixed format message and identifierstherefrom; means for reforming said fixed format message into themessage protocol associated with the communications terminal identifiedin the transmitted message; and means for transmitting said reformattedfixed format message to the identified digital communications terminaltype.
 5. The digital communication apparatus of claim 1 wherein saidfirst and second means comprise:storage buffer means for receiving andstoring fixed format messages and free text messages; and microprocessormeans coupled to said storage buffer means for processing fixed formatand free text messages stored therein, for identifying the message typeof each fixed format digital message stored in said storage buffermeans, for tagging each fixed format digital message with digitalidentifiers indicating the fixed format message protocol type, that thismessage is to be transmitted as a free text message, and the terminaltype to which said fixed format message is to be transmitted, forformatting each tagged fixed format digital message into a predeterminedfree text message format compatible with said communications link, andfor detecting the receipt of free text messages, for unformatting eachfree text message to extract the fixed format message therefrom bydetermining the fixed format message type, determining the terminal towhich the fixed format message is to be sent, and unpacking the fixedformat message from each free text message, and for storing each fixedformat message in said storage buffer means prior to transmittal to theidentified communication terminal.
 6. Digital communication apparatusfor use with secure, jam-resistant digital communication equipment, saiddigital communication equipment having a message protocol which includesa free text message protocol, said digital communication apparatusallowing transfer of data, over said digital communication equipment,between digital communications terminals of a predetermined type havinga fixed format message protocol which is incompatible with the messageprotocol of said digital communication equipment, said digitalcommunication apparatus comprising:first means, adapted to receive fixedformat digital messages having a fixed format message protocolcompatible with said communications terminals, for packing said fixedformat digital message into free text digital messages having a freetext message protocol compatible with the free text message protocol ofsaid jam-resistant digital communication equipment; and second meansadapted to receive free text digital messages compatible with saidcommunication terminal and having a fixed format digital messagecontained therein, for unpacking said fixed format digital message fromsaid free text digital message to provide said fixed format digitalmessages for use with said communication terminals.
 7. The digitalcommunication apparatus of claim 6 wherein said first meanscomprises:means for storing said fixed format digital message receivedfrom said digital communications terminal; means for identifying themessage protocol type of said fixed format digital message; means fortagging said fixed format digital message with digital identifiersindicating the fixed format message protocol type, that this message isto be transmitted as a free text message, and the terminal type to whichsaid fixed format message is to be transmitted; means for formattingsaid tagged fixed format digital message into said predetermined freetext message format compatible with said communications link; and meansfor storing said formatted free text message.
 8. The digitalcommunication apparatus of claim 7 wherein said means for formattingcomprises:means for creating a free text header word which includes saiddigital identifiers; and means for copying said fixed format messageinto unoccupied digital words comprising said free text message.
 9. Thedigital communication apparatus of claim 7 wherein said second meanscomprises:means for storing a free text message received from saidcommunications link; means for detecting the receipt of said free textmessage from said communications link; means for unformatting said freetext message to extract said fixed format message and identifierstherefrom; means for reformatting said fixed format message into themessage protocol associated with the communications terminals identifiedin the transmitted message; and means for transmitting said reformattedfixed format message to the identified digital communications terminaltype.
 10. A method of producing digital message compatible with acommunications link derived from a digital communications terminalemploying a message protocol incompatible with said communications link,said method comprising the steps of:storing said fixed format digitalmessage received from said digital communications terminal; identifyingthe message protocol type of said fixed format digital message; taggingsaid fixed format digital message with digital identifiers indicatingthe fixed format message protocol type, that this message is to betransmitted as a free text message, and the terminal type to which saidfixed format message is to be transmitted; formatting said tagged fixedformat digital message into a predetermined free text message formatcompatible with said communications link; and storing said formattedfree text message.
 11. The method of claim 10 wherein said step offormatting said free text message comprises the steps of:creating a freetext header word which includes said digital identifiers; and copyingsaid fixed format message into unoccupied digital word comprising saidfree text message.
 12. A method of communicating between digitalcommunications terminals employing a fixed message format and protocolusing a communications link employing an incompatible message format andprotocol, said method comprising the steps of:storing a digital messagehaving a predetermined fixed format derived from said digitalcommunications terminal; identifying the message type of said storedfixed format digital message; tagging said fixed format digital messagewith digital identifiers indicating the fixed format message protocoltype, that this message is to be transmitted as a free text message, andthe terminal type to which said fixed format message is to betransmitted; formatting said tagged fixed format digital message into apredetermined free text message format compatible with saidcommunications link; storing said formatted free text message;transmitting said formatted free text message over said communicationslink; storing a free text message received from said communicationslink; detecting the receipt of said free text message from saidcommunications link; unformatting said free text message to extract saidfixed format message and identifiers therefrom; reformatting said fixedformat message into the message protocol associated with thecommunications terminal identified in the transmitted free text message;and transmitting said reformatted fixed format message to the identifieddigital communications terminal type.
 13. The method of claim 12wherein:said step of formatting said free text message comprises thesteps of creating a free text header word which includes said digitalidentifiers, and copying said fixed format message into unoccupieddigital words comprising said free text message; and said step ofunformatting said free text message comprises the steps of determiningthe type of fixed format message contained in said free text message,and removing said fixed format message from the words of said free textmessage.
 14. A method of communicating between digital communicationsterminals employing a fixed message format and protocol using acommunications link employing an incompatible message format andprotocol, said method comprising the steps of:storing a digital messagehaving a predetermined fixed format derived from said digitalcommunications terminal; identifying the message type of said storedfixed format digital message; tagging said fixed format digital messagewith digital identifiers indicating the fixed format message protocoltype, that this message is to be transmitted as a free text message, andthe terminal type to which said fixed format message is to betransmitted; formatting said tagged fixed format digital message into apredetermined free text message format compatible with saidcommunications link; storing said formatted free text message;transmitting said formatted free text message over said communicationslink; storing a free text message received from said communicationslink; detecting the receipt of said free text message from saidcommunications link; unformatting said free text message to extract saidfixed format message and identifiers therefrom; unformatting said freetext message to extract said fixed format message thereform, whichincludes determining the fixed format message type, determining theterminal to which the fixed format message is to be sent, and removingthe fixed format message from said free text message; and transmittingsaid reformatted fixed format message to the identified digitalcommunications terminal type.
 15. The digital communications apparatusof claim 2 further comprising means for coupling a stored formatted freetext message from said means for storing said formatted free textmessage to said communications link.
 16. The digital communicationsapparatus of claim 15 wherein said means for formatting comprises:meansfor creating a free text header word which includes said digitalidentifiers; and means for copying said fixed format message intounoccupied digital words comprising said free text message.
 17. Thedigital communications apparatus of claim 7 further comprising means forcoupling a stored formatted free text message from said means forstoring said formatted free text message to said communications link.18. The digital communications apparatus of claim 17 wherein said meansfor formatting comprises:means for creating a free text header wordwhich includes said digital identifiers; and means for copying saidfixed format message into unoccupied digital words comprising said freetext message.